How do i use Activator.CreateInstance with strings?

问题

In my reflection code i hit a problem with my generic section of code. Specifically when i use a string.

var oVal = (object)"Test";
var oType = oVal.GetType();
var sz = Activator.CreateInstance(oType, oVal);

Exception

An unhandled exception of type 'System.MissingMethodException' occurred in mscorlib.dll

Additional information: Constructor on type 'System.String' not found.

I tried this for testing purposes and it occurs in this single liner too

var sz = Activator.CreateInstance("".GetType(), "Test");

originally i wrote

var sz = Activator.CreateInstance("".GetType());

but i get this error

Additional information: No parameterless constructor defined for this object.

How do i create a string using reflection?

回答1:

Keep in mind that the string class is immutable. It cannot be changed after it is created. That explains why it doesn't have a parameterless constructor, it could never generate a useful string object other than an empty string. That's already available in the C# language, it is "".

Same reasoning applies for a string(String) constructor. There is no point in duplicating a string, the string you'd pass to the constructor is already a perfectly good instance of the string.

So fix your problem by testing for the string case:

var oType = oVal.GetType();
if (oType == typeof(string)) return oVal as string;
else return Activator.CreateInstance(oType, oVal);

回答2:

You are trying to do this :

var sz = new string();

Try to compile it, you will understand your error.

You may try :

var sz = Activator.CreateInstance(typeof(string), new object[] {"value".ToCharArray()});

But it looks useless, you should directly use value...

回答3:

It looks like you're trying to call a constructor which just takes a string - and there isn't such a constructor. If you've already got a string, why are you trying to create a new one? (When you didn't provide any further arguments, you were trying to call a parameterless constructor - which again, doesn't exist.)

Note that typeof(string) is a simpler way to get a reference to the string type.

Could you give us more information about the bigger picture of what you're trying to do?

回答4:

String actually has no constructor that takes a string as input. There is a constructor that takes a char array so this should work:

var sz = Activator.CreateInstance ("".GetType (), "Test".ToCharArray ());

回答5:

This is what I use in my projects. As far as needing to create an instantiation of a type of object and not knowing at design time, is rather normal for me. Perhaps you are cycling through object properties and you want to instantiate all of them dynamically. I have many times needed to create then assign values to non instantiated POCO objects... with the below code you can use a string value stored in the DB to instantiate an object as well or instantiate an object stored in a library that is referencing your library - so you can bypass circular reference errors as well... Hope it helps.

using System;
using System.Collections.Generic;
using System.Globalization;
using System.Reflection;

/// <summary>
/// Instantiates an object. Must pass PropertyType.AssemblyQualifiedName for factory to operate
/// returns instantiated object
/// </summary>
/// <param name="typeName"></param>
/// <returns></returns>
public static object Create(string typeAssemblyQualifiedName)
{
  // resolve the type
  Type targetType = ResolveType(typeAssemblyQualifiedName);
  if (targetType == null)
    throw new ArgumentException("Unable to resolve object type: " + typeAssemblyQualifiedName);

  return Create(targetType);
}

/// <summary>
/// create by type of T
/// </summary>
/// <typeparam name="T"></typeparam>
/// <returns></returns>
public static T Create<T>()
{
  Type targetType = typeof(T);
  return (T)Create(targetType);
}

/// <summary>
/// general object creation
/// </summary>
/// <param name="targetType"></param>
/// <returns></returns>
public static object Create(Type targetType)
{
  //string test first - it has no parameterless constructor
  if (Type.GetTypeCode(targetType) == TypeCode.String)
    return string.Empty;

  // get the default constructor and instantiate
  Type[] types = new Type[0];
  ConstructorInfo info = targetType.GetConstructor(types);
  object targetObject = null;

  if (info == null) //must not have found the constructor
    if (targetType.BaseType.UnderlyingSystemType.FullName.Contains("Enum"))
      targetObject = Activator.CreateInstance(targetType);
    else
      throw new ArgumentException("Unable to instantiate type: " + targetType.AssemblyQualifiedName + " - Constructor not found");
  else
    targetObject = info.Invoke(null);

  if (targetObject == null)
    throw new ArgumentException("Unable to instantiate type: " + targetType.AssemblyQualifiedName + " - Unknown Error");
  return targetObject;
}

/// <summary>
/// Loads the assembly of an object. Must pass PropertyType.AssemblyQualifiedName for factory to operate
/// Returns the object type.
/// </summary>
/// <param name="typeString"></param>
/// <returns></returns>
public static Type ResolveType(string typeAssemblyQualifiedName)
{
  int commaIndex = typeAssemblyQualifiedName.IndexOf(",");
  string className = typeAssemblyQualifiedName.Substring(0, commaIndex).Trim();
  string assemblyName = typeAssemblyQualifiedName.Substring(commaIndex + 1).Trim();

  if (className.Contains("[]"))
    className.Remove(className.IndexOf("[]"), 2);

  // Get the assembly containing the handler
  Assembly assembly = null;
  try
  {
    assembly = Assembly.Load(assemblyName);
  }
  catch
  {
    try
    {
      assembly = Assembly.LoadWithPartialName(assemblyName);//yes yes this is obsolete but it is only a backup call
    }
    catch
    {
      throw new ArgumentException("Can't load assembly " + assemblyName);
    }
  }

  // Get the handler
  return assembly.GetType(className, false, false);
}

来源：https://stackoverflow.com/questions/2092530/how-do-i-use-activator-createinstance-with-strings

标签

.net

reflection

activator